A Study on Non-volatile Copper-oxide Based Conductive Bridge Random-access-memory and Amorphous Chalcogenide Based Selector

Abstract

Recently, 3 dimensional (D) cross-point memory-cell has been a great of interest opening a new non-volatile storage-class-memory covering the memory field between dynamic-random-access-memory and NAND flash memory in the memory hierarchy. The SCM was introduced to improve the inefficiency of the data storage system by reducing the gaps of performance and stability between DRAM and NAND Flash memory. The SCM has a 3D cross-point array structure, and Since each memory cell has a structure in which 1R and 1S are vertically stacked, it is necessary to study both 1R and 1S. I conducted a study on CBRAM, the most promising candidate reported on the 2018 IRDS roadmap, and a metal-filament based threshold-switching selector of a stacked structure that has not been reported previously. First, I investigated how to remove the forming process, one of the biggest problems of CBRAM, and understood the forming-free mechanism using CuxO solid-electrolyte-based CBRAM. Six major scientific and technical discoveries are reported here: (i) the crystalline structure of a CuxO solid electrolyte depended on the temperature of the ex-situ annealing of the solid electrolyte; i.e., cubic crystalline structure below 250C and monoclinic crystalline structure above 300C, ii) electro-forming-free and electro-reset processes could be achieved at a specific ex-situ annealing temperature (i.e., 250C) while no electro-reset process below 200C and an electro-forming process above 300C were necessary, iii) a higher ex-situ annealing temperature led to a lower remaining Cu2+ vacancy concentration in the solid electrolyte, iv) a higher remaining Cu vacancy concentration led to a deeper diffusion depth of Ag atoms in the solid electrolyte, v) a shallower diffusion depth of Ag atoms led to a much more precisely inversely conical shape of Ag-atom filaments in a CuxO solid electrolyte, and vi) both electro-forming and electro-reset processes were achieved at an moderate inversely conical shape of a metal-atom filament. Second, I conducted a research on the super-linear threshold selector using a GeSe chalcogenide material that is frequently used in CBRAM and ovonic threshold switch (OTS). Among chalcogenide materials, the GeSe is attractive due to its environment-friendly, wide bandgap, simple binary composition with high thermal stability and a crystalline temperature of ~380oC. Specifically, due to the wide bandgap characteristics of the amorphous GeSe, leakage current can be reduced and selectivity can be improved because it can have high resistance in the original off-state of the selector device. In addition, since the ionic mobility and diffusivity of Cu or Ag ions in GeSe layer is very fast, the switching speed of CBRAM and selector is fast, and the selector operation is possible at high on-current (> 1MA/cm2). Five major scientific and technical discoveries are reported here: i) the design of new type super-linear threshold switching selector by controlling the Cu-concentration profiles using Cu diffusion barrier, ii) the achievement of forming-free characteristics, adjustable Vth, the selectivity of > 107, endurance cycles of > 106, and switching time of 85 ns using a selector with the Cu-concentration profile of a trident shape, iii) the dependencies of I-V curves on the cell diameter and α-GeSe thickness, iv) the current conduction mechanism for a super-linear threshold selector with the threshold switching and spontaneous rupture characteristics, v) the applicability of the super-linear threshold selector in 1S1R structure for achieving high-density cross-point memory cells.